1. Field of the Invention
The present invention relates to an image heating apparatus suitable for use as a heat fixing apparatus mounted on photocopiers and printers as well as a heater used in the apparatus.
2. Related Background Art
As a heat fixing apparatus mounted on a photocopier or a printer, the one which comprises a flexible sleeve, a ceramic heater brought into contact with the inner surface of the flexible sleeve and a pressure roller forming a nip portion with the ceramic heater by sandwiching the flexible sleeve and is configured to convey recording material carrying toner images with a nip portion and meanwhile bring the toner images into heat fixing onto the recording material has been put into practical use. The heat fixing apparatus (called film heating system) has a very small heat capacity, and therefore is advantageous in terms of short warmingup to reach the fixable temperature to make short a period of waiting for printing and of less power consumption under the state of waiting for a print instruction and the like.
The quality of material for the flexible sleeve is polyimide or stainless. In addition, the ceramic heater is a plate-shaped ceramic substrate excellent in heat-resisting property, heat conducting property and electro-insulating property, made of almina, aluminium nitride and the like, on which a heat generating resistor with silver and paradium as main components is printed. Based on detection temperature of a thermistor brought into contact with this ceramic heater, electrical power supply to the heat generating resistor is controlled to supervise the temperature of the heater.
Such a heat fixing apparatus is provided with safety measures in assumption of the case where a circuit controlling heat dissipation of the ceramic heater ends in giving up normal operation due to some causes. In particular, between the power supply and the heat generating resistor an safety element (heat sensing element) such as a thermoswitch, a temperature fuse and the like are brought into electrical connection and this safety element is brought into contact with the ceramic heater. In the case where the heat generating resistor has run away (in the case where the ceramic heater has given rise to abnormal heat dissipation), the heat from the ceramic heater operates the safety element to open the electric path from the power supply to the heat generating resistor so as to cut off electrical power supply to the heat generating resistor, and thereby abnormal temperature rise of the ceramic heater is prevented. Here, in case of a toner image formed on a small-sized recording material into heat fixing, in the direction perpendicular to the recording material conveyance direction, in the region where recording material passes the heat of the ceramic heater is deprived by the recording material, but in the region where recording material does not pass, the heat of the ceramic heater is not deprived by the recording material and therefore excess temperature might take place (generally called temperature rise in non-paper feeding portion). The safety element is normally disposed within the region where a small-sized recording material passes so that the safety element do not operating by this temperature rise in non-paper feeding portion.
Incidentally, the safety element such as a thermoswitch, a temperature fuse and the like has heat capacitance to a certain extent. Accordingly, in the region where the safety element is brought into contact with the ceramic heater, since the heat is deprived by the safety element, the temperature readily drops. On the contrary, in the region where the safety element is not brought into contact, absence of heat transfer to the safety element readily gives rise to unevenness of temperature distribution between in the region where safety element is brought into contact and in the region where safety element is not brought into contact.
Accordingly, a technique for correcting unevenness of temperature distribution due to existence of a safety element has been disclosed in Japanese Patent Application Laid-Open No. H09-297478. In particular, in the technique, the resistance value of a heat generating resistor in the region where an safety element is brought into contact is made larger than the resistance value of the adjacent region so as to make the heat dissipation amount of the region where an safety element is brought into contact larger than the adjacent region and thereby the heat deprived by the safety element is compensated.
On the other side, sizes of recording material (recording paper) application for use in a photocopier and a printer normally exist in plurality. Especially, in case of bringing a toner image formed on a small-sized recording material into heat fixing, the above described temperature rise in non-paper feeding portion might take place. Excess temperature rise is not preferable since it will result in decreasing endurance property of a heat fixing apparatus, and in case of bringing large-sized paper into fixing in succession to the fixing step on small-sized paper, will result as well in image defects with the toner images ending in hot offset and the like.
Therefore, a heat fixing apparatus in which heat dissipation distribution of a ceramic heater can be changed in accordance with size of recording material has been disclosed in Japanese Patent Application Laid-Open No. H10-177319. The ceramic heater mounted on this heat fixing apparatus has on a ceramic substrate a first heat generating resistor with resistance value in the center in longitudinal direction being larger than those in the both ends thereof and a second heat generating resistor with resistance value in the both ends being larger than in the center, and electrical power supply to these two heat generating resistors are made individually controllable. In this case, the center in longitudinal direction is the conveyance reference of recording material where recording material in all sizes passes. Setting various electrical power supply ratio to the first heat generating resistor and the second heat generating resistor enables setting of various kinds of heat dissipation distributions of the ceramic heater.
Use of the above described safety element can be considered as safety measurements on the ceramic heater having a plurality of heat generating resistors with different heat dissipation distributions. In addition, also in this heater, in order to prevent the safety element from mal-operation due to temperature rise in non-paper feeding portion as described above, it can be considered that the safety element is disposed within a region where a small-sized recording material passes, that is, a region of the first heat generating resistor where the heat dissipation amount is large.
In assumption of such a runaway pattern on the heat generating resistors in the heat fixing apparatus, firstly, in case of the both of two heat generating resistors having run away, naturally, the safety element will operate quickly to enable prevention of abnormal temperature rise. Next, in the case where only the first heat generating resistor has run away, since the safety element is disposed in the region of the first heat generating resistor where the heat dissipation amount is large, likewise the safety element will operate quickly to enable prevention of abnormal temperature rise.
However, in the case where only the second heat generating resistor has run away, since the safety element is disposed indeed in the region of the first heat generating resistor where the heat dissipation amount is large, but in the region of the second heat generating resistor where the heat dissipation amount is small, it can be considered that responsiveness of the safety element gets bad.